The goals of this project are to improve understanding of how femoral stem design effects cement mantle failure in total hip replacements (THR) and to validate a pre-clinical test for cemented femoral stems. THR has been a very common procedure for some time. Cemented femoral components, which rely on Polymethylmethacrylate (PMMA) for fixation of the stem within the bone, account for about half of the THR done in the USA. Worldwide, cemented stems are used far more commonly. Despite the widespread and ongoing use of PMMA cement for femoral stem fixation, the influence of stem design on cement mantle failure is poorly understood. Many 'improved' designs of cemented femoral stem have been introduced in recent years but with mixed clinical results. That novel stem designs have caused such surprises is due, in part, to the lack of an in vitro analogue that can accurately model the medium to long-term response to cyclic loading of the stem/cement/femur system. The goal of this project is to develop and validate such an in vitro analogue. A physical model is proposed by which the medium to long-term fatigue response of a PMMA cement mantle can be simulated in cadaveric tissue. The key feature of the model is formulation of cement with degraded fatigue fracture properties (sub-cement) such that long-term fatigue tests can be shortened by two orders of magnitude. Specific Aims: 1 To formulate sub-cement, which will be similar to conventional PMMA bone cements in terms of modulus, yield stress, and viscosity but will be substantially more susceptible to fatigue crack propagation. 2 To demonstrate that cemented femoral stems with lower and higher risks of medium to long term aseptic loosening can be differentiated by analysis of in vitro fatigue damage and stem/femur micro-motions in a sub-cement based stem/cement/femur construct. 3 To demonstrate that damage accumulation patterns in sub-cement based stem/cement/femur constructs correlate with patterns found in post-mortem retrievals. Should these aims be met, the proposed model will be of great use in pre-clinical testing of novel cemented stem designs.